An essential element of modern mobile communications systems is the “cell site.” The cell site includes one or more directional base station antennas aimed at a desired geographical area of coverage with coaxial cable connecting the antennas to base station radio equipment. The base station radio equipment acts as a transceiver, transmitting high power signals (37 dBm to 49 dBm channel power) in the direction of mobile users and receives low power signals (as low as −106 dBm channel power) from mobile user equipment. Often the range of a cell site is uplink limited, meaning that the distance that a particular cell site is able to cover is limited by the transmit power of the mobile equipment and the path loss between the mobile equipment and the cell site. The signal received at the cell site from mobile equipment reduces by 6 dB each time the distance between the mobile equipment and the cell site is doubled. Eventually, the signal from the mobile equipment drops to a level where the base station receiver is not able to distinguish the signal from background noise, and the call is dropped.
Mobile operators typically design their network of cell sites to achieve a signal-to-interference plus noise ratio (SINR) of >6 dB across the region of coverage. This means that the signal level arriving from mobile equipment at each base station receiver should be at least 6 dB higher than all sources of noise generated within or received by the base station receiver. For data networks, the network of cell sites may be designed to achieve even higher SINR levels to enable faster data transmission.
In order to achieve the strongest possible signal from mobile equipment, it is desirable to locate the cell site antennas at an elevation that is higher than the average clutter (trees, buildings, etc.) in the area to be covered. This can be achieved by erecting a tower at the cell site to support the antennas at the desired elevation or can be achieved by mounting the antennas on existing infrastructure, such as building rooftops. When mounting antennas on buildings, the building owner may require for aesthetic reasons that the operator only install the antennas at locations that are not visible from the ground outside the building. This often forces the antennas to be installed near the center of the building rooftop rather than near the edges of the building.
For example, FIG. 1 shows a ground-level perspective view of a building 100, showing that no cell site is visible. However, FIG. 2 shows building 100 from an upper oblique view, revealing cell site 200 on roof 201, set back from the edges of roof 201.
The performance of cell sites installed on building rooftops is often limited due to passive intermodulation (PIM). Passive intermodulation occurs when the high-power downlink signals broadcast by the base station antennas mix at passive, non-linear junctions in the RF path, creating new signals. If these new signals (intermodulation products) fall in an operator's uplink band, they act as interference and reduce SINR. As the SINR is reduced, the geographic coverage of the cell site is reduced and the data capacity of that cell site is reduced.